Transistor bias and temperature compensation circuit

ABSTRACT

A NOVEL TRANSISTOR AMPLIFIER BIAS AND TEMPERATURE COMPENSATION CIRCUIT IS DISCLOSED EMPLOYING IN THE INPUT CIRCUIT A CURRENT-DIVISION NETWORK ONE PATH OF WHICH EMBODIES A DIODE OR THE LIKE CONNECTED BETWEEN THE BASE AND EMITTER OF THE AMPLIFIER.

. Feb. 23, 1971' D. R. VON RECKLINGHAUSEN ,5

TRANSISTOR BIAS AND TEMPERATURE COMPENSATION CIRCUIT Original Filed.Dec. 21, 1964 OUTPUT I \NVENTCR DYANHEL Rm RECKUNGHAUSEN AT TO RN EYSUnited States Patent ABSTRACT OF THE DISCLOSURE A novel transistoramplifier bias and temperature compensation circuit is disclosedemploying in the input circuit a current-division network one path ofwhich embodies a diode or the like connected between the base andemitter of the amplifier.

5 Claims This application is a continuation of Ser. No. 419,888, filedDec. 21, 1964, now abandoned.

The present invention relates to transistor amplifier circuits, and morespecifically to circuits adapted for automatic temperature compensationwith simultaneous bias adjustment and flexibility for use withtransistor amplifiers of different manufacturing tolerancecharacteristics, together with minimal external influence on furtherstages associated with the amplifiers.

Transistor amplifier circuits have previously been provided withvariable bias input circuits employing potentiometers and other variableresistance devices, though most commonly no adjustable bias has beenprovided. Many of such devices, however, do not provide temperaturecompensation for variations in transistor properties with ambienttemperature changes. Suggestions have accordingly been made toincorporate diodes and other negative temperature coefficient ofresistance devices that, when subjected to the same ambient temperaturevariations as the transistor amplifier, will themselves exhibitresistance changes compensatory of the resistance changes effected inthe transistor amplifier. While a measure of temperature compensation isthereby attained, these two-terminal negative temperature coeflicient ofresistance diodes and the like are not themselves adjustable forvariation in their output which supplies transistor bias. That biasmust, moreover, accommodate the relatively wide range of differentcharacteristics of transistors introduced by the tolerances inmanufacture of the same and in the use of transistors of different gainand other characteristics in the same circuit. Designers haveaccordingly usually chosen the lesser of the two disadvantageous resultsabove-described in designing circuits of this character; namely, a diodeor thermistor in an invariant circuit, selected in the laboratory withsuch a value as to represent a mean of the production variationencountered in practice.

In accordance with the present invention, it has been discovered that,through a novel circuit and rather simple adjustment of relativeparameters, negative temperature coeificient of resistance devices suchas silicon or other solid-state diodes or the like, preferably thoughnot always essentially matched to the type of solid state mate rial ofthe transistor amplifier, are, in effect, made variable in the sensethat they can provide not only temperatore-compensating effects in theinput circuit of a transistor amplifier, but enable variation of theoutput thereof to satisfy the requirements of production variances indifferent transistor amplifiers; and, at the same time,

through the employment of a constant-current circuit connection, canexert minimum influence on any previous or external amplifier circuitsor loads associated with the transistor amplifier.

A further object of the invention is to provide a new and improvedtransistor amplifier.

Other and further objects will be explained hereinafter and will be moreparticularly pointed out in the appended claims.

It will be noted that transistors of wider tolerances and diodes orother two-terminal negative temperature coefficient of resistancedevices of wider tolerances may thus be employed in amplifier circuitsin accordance with the present invention than has heretofore beenpossible.

The invention will now be described in connection with the accompanyingdrawing:

FIG. 1 of which is a schematic circuit diagram of a single amplifierstage illustrating certain of the features of the invention;

FIG. 2 is a similar diagram of a preferred modification; and

FIG. 3 is a circuit diagram illustrating the invention employed inconnection with a transformer input signal application means.

Referring to FIG. 1, a transistor amplifier is shown at 1 comprising abase electrode 3, a collector electrode 5 and an emitter electrode 7that, in the illustrated embodiment, is connected (optionally) through aresistance R to a terminal 2. The current supply for providing bias forthe transistor amplifier stage 1 is shown at and respectively connectedthrough a resistor R and by a conductor 2 to the input circuit 4 betweenthe base electrode 3 and the emitter electrode 7. The input circuit 4 isshown comprising an energy-divider variable resistance potentiometer 6the slider S of which may introduce more or less resistance to the leftor right thereof for the bias variation purpose before discussed andlater more fully explained. The righthand terminal of the potentiometer6 is shown connected to the base electrode 3 and to one side of atwo-terminal negative temperature coefficient of resistance device Dsuch as one of the solid state diodes beforementioned. The diode D orthe other negative temperature coefficient of resistance device mustoperate in the forward or conducting mode to correspond to the nature ofthe operation of the junction between the base 3 and emitter 7 of thetransistor amplifier 1. The other terminal of the diode D is shownconnected to the terminal 2. The left-hand terminal of the potentiometer6 moreover is preferably connected (though not essentially) through aresistance R to the terminal 2 such that the elements 6-D-R constitutethe input circuit to the amplifier 1.

In accordance with the present invention, it has been found that if thevalue of the resistance R in the path of the current being supplied fromthe terminal to the input circuit 4 is made very large compared with theeffective resistance presented to the current by the input circuit 4(i.e. the resistance of the potentiometer 6 and the elements D and R andthe input impedance of the transistor between the base 3 and. theterminal 2), that substantially constant current will flow through theinput circuit 4 between the terminals and for a reason that will be moreclearly evident in connection with the discussion of FIGS. 2 and 3.Succinctly stated, however, that reason resides in the desirability ofexerting minimum influence on any circuit external to the amplifier 1.

Under the circumstances above set forth, with the collector 5 returnedthrough some desired load L to the terminal the temperature variationsoccurring in the transistor 1 will be compensated for by variations inthe bias applied between the base 3 and the emitter 7 throughcorresponding temperature-produced variations in the diode D. Inaddition, effective variation in the output of the diode D throughvarying its effective resistance is provided by means of the movement ofthe potentiometer slider 5 in the constant current circuit betweenterminals and through the input circuit 4, such variation enabling thematching of the desired bias characteristics to transistors of wideranges of differences in properties and production tolerances.

1n the circuit of FIG. 2, similar input circuits are shown applied to apair of push-pull transistors 1 and 1, there being two diodes D and D,both connected in the forward conducting mode for the reasons beforediscussed. The reason for two diodes resides in the fact that there aretwo junctions that must be temperature compensated; namely, the onebetween the base 3 and emitter 7 of the transistor stage 1 and the otherbetween the base 3 and emitter 7' of the other push-pull stage 1. Thesame relative values of the input circuit components, however, arerequired in the system of FIG. 2. to produce the results before stated.The diodes D and D are effectively connected in the transistor amplifierinput circuit between base and emitter electrodes, analogous to theconnections of the single-ended amplifier of FIG. 1. The input signalapplied to the bases of the push-pull amplifiers 1 and 1 is provided byan input stage 10 the collector current of which, applied from terminal2 to the collector 15, is maintained constant by the constant currentinput supply before referred to. Minimum change in amplification oroverload of the driver stage 10 to which the input signal is applied atthe base 13 is thus attained by this constant current path, he emitter17 being shown returned to the or ground terminal in this circuit. Thus,minimum influence is exerted by the temperature compensation andvariable bias circuit of the present invention upon external circuits.The output of the push-pull amplifier, which may, if desired, throughappropriate operation of a slider S be operated anywhere from class A toclass B or beyond, is shown taken through the coupling capacitor C andfrom the or grounded terminal at the emitter 5'. One may, of course,substitute other combinations of stages that perform similar functionsto the push-pull amplifier 1 and 1', if desired, including furtherstages associated in pairs or other multiples with the stages 1 and 1.

While the circuit of FIG. 2 is shown comprising the driver transistorstage 10, furthermore, it is to be understood that other types of inputcircuits may also be beneficially employed with the invention such asthe input push-pull transformer T of FIG. 3. The primary winding P ofthe transformer T receives the input signal and the secondary windings Sand 8;; are respectively connected between the base electrodes 3 and 3'and the respective upper terminals of the compensating diodes D and D'.The secondaries S and S as well as the other components of the circuits,will be substantially symmetrical unless assymetry is required in thebias connections on the similar potentiometers 6 and 6' to produce thebalanced output. In FIG. 3, therefore, a pair of similar input circuits4 and 4' is shown, the circuit 4' having the same circuit elements asthe input circuit 4 but indicated with a prime notation.

In the circuit of FIG. 3, transistors of the same type such as, forexample, NPN or PNPs may be used in both stages 1 and 1'; whereas in thecircuit of FIG. 2, transistors of opposite characteristics such as NPNand PNP will be required for the respective stages 1 and 1'. To maintainsymmetry oi operation of the circuit of FIG. 3 at all operation levelsand divide in half the supply voltage during no input signal and allsignal conditions, the constant current result is attainedsimultaneously with providing the variable temperature sensitive diodeor other device D, enabling not only temperature compensation butvariation of the bias conditions to accommodate amplifiers 1 and 1 ofvarying parameters and characteristics. In conventional nominally classB push-pull amplifiers of this character, for example, the currentthrough the bias networks may readily be several times the zero inputsignal current through the push-pull transistor. In accordance with thepresent invention, any change in the variation of the potentiometers 6and 6 will not make any substantial change in the voltage division toany upper and lower stages. This insures that the bias of the outputamplifier stages may be adjusted in such a manner so that minimumdistortion in amplification results.

Further modifications will occur to those skilled in the art and allsuch are considered to fall within the spirit and scope of the inventionas defined in the appended claims.

What is claimed is:

1. A transistor amplifier provided with base, emitter and collectorelectrodes and having, in combination, a negative temperaturecoefficient two-terminal resistive device connected between the base andemitter electrodes and disposed to be subjected to substantially thesame temperature variations as the amplifier, a variable resistanceelement, means for connecting the said element with the said device tocomprise an input circuit for the amplifier having a pair ofparallel-connected branch paths, only one of which includes said device,means including a resistive path connected to said input circuit forapplying bias current to said input circuit and to divide the currentbetween the said branch paths of the said input circuit, the resistanceof the said resistive path being large compared to the resistance ofsaid input circuit in order to enable variation of the current throughand hence the effective resistance of the said device, and thus enablevariation of bias for the amplifier to be effected simultaneously withcompensation for temperature variations in the amplifier and withsubstantially constant bias current through the said input circuit, saidvariable resistance element comprising means for controlling theproportion of the bias current passed through said parallel-connectedpaths, respectively, of said input circuit.

2. An amplifier as claimed in claim 1 and in which a further transistoramplifier and negative temperature coefficient of resistance device areprovided connected as claimed in claim 1, the first-named and furtheramplifiers being connected in push-pull with their emitters connectedtogether and with the first-named and further resistance devicesconnected in series between the base electrodes of the amplifiers.

3. A pair of amplifiers as claimed in claim 1 means for connecting thesame in push-pull and means for applying signals to the same inpush-pull.

4. A transistor amplifier provided with base, emitter and collectorelectrodes and having, in combination, a negative temperaturecoefficient two-terminal resistive device connected between the base andemitter electrodes and disposed to be subjected to substantially thesame temperature variations as the amplifier, a current-divider networkhaving a variable resistance element, means for connecting the saidelement with the said device to comprise an input circuit for theamplifier, means including a resistive path connected to said inputcircuit for applying bias current to pass the same through said elementand through the said device of the said input circuit, the resistance ofthe said resistive path being large compared to the resistance of saidinput circuit in order to enable variation of the current through andhence the effective resistance of the said device, and thus enablevariation of bias for the amplifier to be effected simultaneously withcompensation for temperature variations in the amplifier and withsubstantially constant bias current through the said input circuit, saidcurrent-divider variable resistance clement comprising a potentiometer,the terminals of which are connected to opposite terminals of saidresistive device.

5 6 5. An amplifier as claimed in claim 4 zmd in which the OTHERREFERENCES F i stohd'stafiidevlce of the Same Herscher: DesigningTransistor Power Amplifiers, Elecma eria as t e sal ransis or ampi er.tronics, 11, 1958, p11

References Cited 5 ROY LAKE, Primary Examiner UNITED STATES PATENTS L.J. DAHL, Assistant Examiner 3,050,644 8/1962 Irons1de 330-24X 3,302,1241/1967 Dix 330-23 US. Cl. XJR.

FOREIGN PATENTS 1U 2 764,154 12/1956 Great Britain 330-24X

